This invention relates to a full color xerographic printing system, either double pass or single pass, with a single polygon, single optical system Raster Output Scanning (ROS) system, and, more particularly, to a dual wavelength laser diode source for the ROS which images the dual beams at a single station as closely spaced spots or at two stations as closely spaced spots on a dual layer photoreceptor with each photoreceptor layer sensitive to only one of the two wavelengths or with one photoreceptor layer sensitive to both wavelengths and located beneath the other layer which absorbs one of the two wavelengths and permits only the second wavelength of light to reach the lower layer. Hence, although sensitive to both wavelengths, the lower layer is only accessed by one of the two wavelengths.
In xerographic printing (also called electrophotographic printing), a latent image is formed on a charged photoreceptor, usually by raster sweeping a modulated laser beam across the photoreceptor. The latent image is then used to create a permanent image by transferring and fusing toner that was electrostatically attracted to the latent image, onto a recording medium, usually plain paper. While other methods are known, the most common method of sweeping the laser beam is to deflect it from a rotating mirror. A multifaceted, rotating polygon mirror having a set of related optics can sweep the beam or sweep several beams simultaneously. Rotating polygon mirrors and their related optics are so common that they are generically referred to as ROSs (Raster Output Scanners), while printers that sweep several beams simultaneously are referred to as multispot printers.
When a xerographic printer prints in two or more colors, it requires a separate latent image for each color printed, called a system color. Color prints are currently produced by sequentially transferring overlapped images of each system color onto an intermediate transfer belt that is passed multiple times, once for each system color, over the photoreceptor. The built-up image is then transferred to a single recording medium and fused. Such printers are called multiple pass printers.
Conceptually, one can build up multiple colors on a photoreceptor or intermediate transfer belt that is passed through the system only once, in a single pass, by using a sequence of multiple xerographic stations, one for each system color. The built-up image on the photoreceptor or ITB can be transferred to a recording medium in a single pass. Additionally, tandem xerographic stations can sequentially transfer images directly to the recording medium in a single pass. Such a printer, called a multistation printer, would have a greater output than a multipass printer operating at the same raster sweep speed because the rasters for each color are operating simultaneously in the single pass printer. However, the introduction of multistation printers has been delayed by 1) cost problems, at least partially related to the cost of multiple xerographic stations and the associated ROSs, and 2) image quality problems, at least partially related to the difficulty of producing similar spots at each imaging station and subsequently registering (overlapping) the latent images on the photoreceptor(s), transfer medium(s) or recording medium(s).
In the practice of conventional bi-level xerography, it is the general procedure to form electrostatic latent images on a xerographic surface by first uniformly charging a charge retentive surface such as a photoreceptor. The charged area is selectively dissipated in accordance with a pattern of activating radiation corresponding to desired images. The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the areas not exposed by radiation.
This charge pattern is made visible by developing it with toner by passing the photoreceptor past a single developer housing. The toner is generally a colored powder which adheres to the charge pattern by electrostatic attraction. The developed image is then fixed to the imaging surface or is transferred to a receiving substrate such as plain paper to which it is fixed by suitable fusing techniques.
Modern business and computer needs often make it advantageous and desirable to reproduce or print originals which contain two or more colors. It is sometimes important that the copy reproduced or printed also contain two colors.
Several useful methods are known for making copies having plural colors. Some of these methods make high quality images, however, there is need for improvements. In particular, it is desirable to be able to print images having two or more highlight colors rather than being limited to a single highlight color. It is also desirable to be able to produce such images in a single pass of the photoreceptor or other charge retentive surface past the printing process areas or stations.
One method of producing images in plural (i.e. two colors, black and one highlight color) is disclosed in U.S. Pat. No. 3,013,890 to W. E. Bixby in which a charge pattern of either a positive or negative polarity is developed by a single, two-colored developer. The developer comprises a single carrier which supports both triboelectrically relatively positive and relatively negative toner. The positive toner is a first color and the negative toner is of a second color. The method develops positively charged image areas with the negative toner and develops negatively charged image areas with the positive toner. A two-color image occurs only when the charge pattern includes both positive and negative polarities.
Plural color development of charge patterns can be created by the method disclosed by F. A. Schwertz in U.S. Pat. No. 3,045,644. Charge patterns are developed of both a positive and negative polarity. The development system is a set of magnetic brushes, one of which applies relatively positive toner of a first color to the negatively charged areas of the charge pattern and the other of which applies relatively negative toner to the positively charged areas.
U.S. Pat. No. 3,816,115 to R. W. Gundlach and L. F. Bean discloses a method for forming a charge pattern having charged areas of a higher and lower strength of the same polarity. The charge pattern is produced by repetitively charging and imagewise exposing an overcoated xerographic plate to form a composite charge pattern.
As disclosed in U.S. Pat. No. 4,403,848, a multi-color printer uses an additive color process to provide either partial or full color copies. Multiple scanning beams, each modulated in accordance with distinct color image signals, are scanned across the printer's photoreceptor at relatively widely separated points, there being buffer means provided to control timing of the different color image signals to assure registration of the color images with one another. Each color image is developed prior to scanning of the photoreceptor by the next succeeding beam. Following developing of the last color image, the composite color image is transferred to a copy sheet. In an alternate embodiment, an input section for scanning color originals is provided. The color image signals output by the input section may then be used by the printing section to make full color copies of the original.
Proposed prior art multistation printers have usually included individual ROSs (each comprised of separate polygon mirrors, lenses, and related optical components) for each station. For example, U.S. Pat. Nos. 4,847,642 and 4,903,067 to Murayama et al. involve such systems. Problems with these systems include the high cost of producing nearly identical multiple ROSs and the difficulty of registering the system colors.
A partial solution to the problems of multistation xerographic systems with individual ROSs is disclosed in U.S. Pat. No. 4,591,903 to Kawamura et al. The '903 patent, particularly with regards to FIG. 6, discusses a recording apparatus (printer) having multiple recording stations and multiple lens systems, but only one polygon mirror. With only one polygon mirror and associated drive motor, the cost of the system is reduced. However, differences in the lenses and mirror surfaces still could cause problems with color registration.
Another approach to overcoming the problems of multistation printers having individual ROSs is disclosed in U.S. Pat. No. 4,962,312 to Matuura et al. The '312 patent illustrates spatially overlapping a plurality of beams using an optical beam combiner, deflecting the overlapped beams using a single polygon mirror, separating the deflected beams using an optical filter (and polarizers or additional filters if more than two beams are used), and directing the separated beams onto associated photoreceptors. The advantage of overlapping the laser beams is a significant cost reduction since the ROS is shared.
However, an actual embodiment of the '312 apparatus would be rather complicated and expensive, especially if four system colors are to be printed. The use of optical beam combiners to overlap beams so that they have similar optical axes would be difficult, expensive, and time consuming. Obtaining similar sized spots on each photoreceptor would also be difficult as it would be difficult to establish the same optical path lengths for each beam. Finally, it would also be difficult to ensure that the latent images on the photoreceptors are registered. Each of these problems is at least partially related to the relative positions of the laser sources.
In U.S. Pat. No. 5,243,359, commonly assigned with the present application and herein incorporated by reference, the multiwavelength closely spaced diode laser sources of the ROS of the color printing system are passed through a common optical system including polygon and then separated by filters to address multiple xerographic stations simultaneously.
This concept enables savings in both space and cost in color applications since several optical systems are combined into one, for example in a four color system a single polygon ROS would be used instead of four.
The registration problems associated with color xerographic printing are addressed by the multiple wavelength single polygon with split beams from the point of view of scanning bow and wobble errors. The use of a single polygon and associated optical system should minimize relative scanning bow and wobble errors at the separate imaging stations.
This concept of a multiwavelength single polygon ROS addresses the general need for compact color printing systems to take advantage of the exploding color printing market. However there is still a strong need to reduce the size and complexity of color printing systems even further. There is also a need to improve the registration errors inherent in xerographic color printing systems which use tandem drums, intermediate transfer belts and multiple passes.
A single pass xerographic printing system does provide approximately twice the speed of a double pass xerographic printing system. A double pass xerographic printing system provides a more compact ROS and a smaller xerographic printing system architecture which avoids needless duplication of printing elements (i.e. developer stations).
It is an object of this invention to provide a dual wavelength beam laser source for a single polygon, single optics, ROS for use in a full color xerographic printing unit.
It is another object of this invention to provide a dual wavelength sensitive, dual layer photoreceptor for use with a single polygon, single optics, ROS in a full colorxerographic printing unit.
It is another object of this invention to provide a dual wavelength sensitive, dual layer photoreceptor xerographic printing system which can be either double pass or single pass.